body {
  margin: 0;
  place-items: center;
  min-width: 320px;
  user-select: none;
  -moz-user-select: none;
  /* Firefox私有属性 */
  -webkit-user-select: none;
  /* Chrome、Safari私有属性 */
  -ms-user-select: none;
  /* IE 10+ 私有属性 */
}

#app {
  margin: 0 auto;
  text-align: center;
  overflow: hidden;
  position: relative;
}

* {
  box-sizing: border-box;
}